1. Field of Endeavor
This disclosure relates generally to fuel systems and more particularly to a system and process for extraction of an auxiliary fuel stream with low concentrations of sulfur compounds from a primary fuel stream having higher concentrations of sulfur compounds including an internally heated module.
2. Description of Related Art
Continuing improvements in the performance, cost and durability of fuel cell systems have continued to raise interest in their use as auxiliary power units (APU), for example, in a vehicle such as a hybrid or fuel cell powered automobile. One limitation to their practical application involves the logistic fuels or conventional fuels used, for example, diesel and kerosene cuts. These fuels have sulfur content in the range of 30 to 3,000 ppm, which either impacts the conversion of these fuels to hydrogen or the synthetic gas stream, or impacts the performance of the fuel cell downstream. One alternative is to require a synthetic “no sulfur” type liquid fuel such as Fischer-Tropsch liquid or gas-to-liquid products for the APU, but this forces the need for two separate fuel streams to be provided for the same vehicle.
Some governmental agencies of the United States for example, have mandated lower levels of sulfur in fuels such as the recent push toward 30 to 300 ppm. Even with levels near the lowest limit, the sulfur content affects the performance of catalysts in down stream processes, and therefore, preventing effective use of fuel cell systems. Technologies, including membrane based technologies, are being developed for refinery scale applications to achieve these lower sulfur specifications. Membrane technologies and specifically prevaporation membrane technology are commercial technologies that are practiced in a number of industries for the separation of higher vapor pressure components from a mixture of liquid stream.
Typical prevaporation assemblies operate from about 90° to about 130° C. and permeate flux generally increases with increasing temperature (heat causes membrane swelling. A vaporizes permeate, upon entering a vacuum space within an assembly, losses heat needed for phase to the leaf (or layered) material of the assembly, as the vapor passes through the length of the assembly. Once the feed solution has cooled significantly, the membrane generally contracts thereby becoming resistant to diffusion which reduces the membrane module fractionation efficiency/capacity.
U.S. Pat. No. 5,445,731 describes reheating retentate external to the apparatus before reintroducing it into a second membrane module.
Plate and frame designs heating elements are described in U.S. Pat. No. 4,650,574 to provide heating within a membrane assemblies which have thermal resistance between the heating media and the feed solution.
It would be desirable to have a simple process that provides the needed separation selectivity, that could be implemented in a compact, inexpensive system and that would operate with a wide range of primary fuels from gasoline, kerosene, jet fuel and diesel or water-soluble mixtures. It is to these needs that the present disclosure is directed.